a judd illusion in far-aiming: evidence of a contribution to action by vision for perception
TRANSCRIPT
Exp Brain Res (2009) 197:199–204
DOI 10.1007/s00221-009-1889-zRESEARCH NOTE
A Judd illusion in far-aiming: evidence of a contribution to action by vision for perception
John van der Kamp · Hemke van Doorn · Rich S. W. Masters
Received: 26 September 2008 / Accepted: 28 May 2009 / Published online: 14 June 2009© The Author(s) 2009. This article is published with open access at Springerlink.com
Abstract The present study addresses the role of visionfor perception in determining the location of a target in far-aiming. Participants (N = 12) slid a disk toward a distanttarget embedded in illusory Judd Wgures. Additionally, in aperception task, participants indicated when a movingpointer reached the midpoint of the Judd Wgures. The num-ber of hits, the number of misses to the left and to the rightof the target, the sliding error (in mm) and perceptual judg-ment error (in mm) served as dependent variables. Resultsshowed an illusory bias in sliding, the magnitude of whichwas comparable to the bias in the perception of target loca-tion. The determination of target location in far-aiming isthus based on relative metrics. We argue that vision for per-ception sets the boundary constraints for action and thatwithin these constraints vision for action autonomouslycontrols movement execution, but alternative accounts arediscussed as well.
Keywords Ventral system · Dorsal system · Far-aiming · Judd-illusion
Introduction
Goodale and Milner’s (1992) proposal that the dorsal andventral systems can be distinguished by the functional
demands that they serve in action and perception, has expli-cated a confusion of neurophysiological and behavioralobservations. Nevertheless, contributions to action by theventral system, if any, are not clearly understood (Glover2004; Milner and Goodale 2008). Hence, rather thanaddressing the distinction between vision for perceptionand vision for action,1 we aimed to assess the role of visionfor perception in the course of action. To this end, weinvestigated a far-aiming task that was modeled on a tradi-tional Dutch sport called shuZe-boarding (‘sjoelen’), inwhich players slide disks toward a distant target.
According to Milner and Goodale vision for perceptionserves to obtain knowledge about the environment, usinginformation that speciWes objects and their properties inrelation to surrounding objects in relative metrics. They fur-ther argued that vision for action supports movement con-trol and relies on information that speciWes objects inabsolute metrics. Research using visual illusions has shownthat perception of object properties, such as size and loca-tion, depends strongly on visual context, attesting to the useof relative metrics. Movement control, in contrast, remainsrelatively unaVected by visual context, suggesting thatabsolute metrics are used (Agliotti et al. 1995; Ganel et al.2008). The role of vision for perception, however, is notconWned to perception. Milner and Goodale (2008) (van derKamp et al. 2008) argued that the perception or identiWca-tion of action goals and the selection of an appropriateaction entail key contributions from vision for perception.For example, van Doorn et al. (2007) (see also Crajé et al.
J. van der Kamp (&) · H. van DoornFaculty of Human Movement Sciences, Research Institute MOVE, VU University Amsterdam, Van der Boechorststraat 9, 1081 BT Amsterdam, The Netherlandse-mail: [email protected]
J. van der Kamp · R. S. W. MastersInstitute of Human Performance, University of Hong Kong, Hong Kong SAR, China
1 We only obtained behavioral data. Hence, our claims are restricted tobehavior and are necessarily suggestive with respect to the underlyingneural circuitry. We use ‘vision for perception’ and ‘vision for action’to refer to the behavioral processes of detecting and using visual infor-mation for perception and action.
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2008) recently demonstrated that in picking up relativelylarge objects the choice of either a one- or a two-handedgrasp is aVected by an illusion. The control of handaperture, however, remained unaVected. In short, vision forperception and action appear to serve distinct yet comple-mentary functions in action. Vision for perception sets theboundary constraints for action, and within these con-straints the movements are autonomously controlled byvision for action (Milner and Goodale 2008).
The present study addresses the roles of vision for per-ception and action in aiming toward a target. In particular,we asked what type of information is used to determine thelocation of a target. One conjecture is that target location isspeciWed relative to its context. That is, vision for percep-tion determines the target location relative to adjacentobjects in the environment and sets this location as theboundary constraint for action. Informed by vision for per-ception, vision for action subsequently exploits (egocentric)information that speciWes this target location in absolutemetrics to the actor (Gentilucci et al. 1996). Inaccurate aim-ing would reXect errors in the initial determination of targetlocation by vision for perception. An alternative conjectureis that vision for action independently (i.e., without engage-ment of vision for perception) uses instantaneous informa-tion pertaining to the target location in relation to the actor,without taking the broader visual context into account. Formovement control, this may be more eYcient than relyingon ‘delayed’ target location information provided by visionfor perception (Westwood and Goodale 2003).
Unlike far-aiming, near-aiming, especially pointing, iswell-researched in this context. In the typical study, partici-pants make pointing movements to the endpoints of a lineembedded in illusory surroundings, such as a Müller-LyerWgure (e.g., Gentilucci et al. 1996). A recent review of theextant literature on pointing revealed that under full-visionconditions (i.e., with both the target and the hand in view)endpoint accuracy is largely immune to illusory conWgura-tions. Only when vision of the target is removed (particu-larly before movement onset) does signiWcant movementbias occur (Bruno et al. 2008). In full vision, however, aim-ing reXects movement control that is based on target loca-tion information that is coded egocentrically in absolutemetrics (e.g., the gap between hand position and targetlocation). This online control is attributed to vision foraction.
Importantly, in near-aiming, control of hand position ispossible until contact is made with the target. Hence, analy-sis of only endpoint accuracy (as in Bruno et al.’s (2008)review) leaves doubts about whether determination of tar-get location engages vision for perception. It would bemore convincing to study pointing with no opportunity forparticipants to view their hand, or the target, during move-
ment execution (Westwood and Goodale 2003). In visualopen-loop conditions, initial errors in perceived target loca-tion cannot be annulled by online vision for action controlas the movement unfolds. Yet, these conditions still allowpropriocepsis to reduce pointing error. Moreover, Gentilucciet al. (1996) reported that in full vision the Müller-LyerWgure aVected the entire kinematics of the pointing move-ment (although the precise scope of the bias is diYcult toquantify). This observation led Gentilucci et al. (1996) tospeculate that initial aiming was based on informationcomprising visual context.
Contrary to near-aiming, in far-aiming (e.g., sliding adisk toward a distant target) movement control (bothvisual and proprioceptive) is necessarily conWned to themoment the object is released. Hence, the issue of the typeof information that (i.e., absolute or relative) is used todetermine target location may be more appropriatelyresolved using far- rather than near-aiming tasks. Twostudies speak to this point, but provide ambiguous results.Glover and Dixon (2004) had participants step and hopfrom one end of a Müller-Lyer Wgure to the other endunder full vision. A small illusion eVect (<3%) was dis-cerned, but its magnitude might have been reduced by themoving body occluding the target, especially in the step-ping task. More recently, van der Kamp and Masters(2008) reported that throwing accuracy in handball is inXu-enced by the goalkeeper’s posture. Notably, the goalkeepermimicked an amputated Müller-Lyer Wgure by raising thearms skyward (i.e., outward Müller-Lyer Wgure), stretch-ing them to the side (i.e., neutral Wgure) or pointing themdownward (i.e., inward Wgure). These goalkeeper posturesaVected the perceived size of the goalkeeper in accordancewith the Müller-Lyer illusion. Intriguingly, participantsthrew the ball further from the goalkeeper when the armswere raised skywards than when the arms were stretched tothe side. However, when the arms were pointing down-ward, the ball was not thrown closer to the goalkeeper.Although this might suggest a reliance on visual context, itcannot be ruled out that the eVects of goalkeeper postureon throwing accuracy are related to factors other than theillusory size of the goalkeeper (e.g., the arms up goal-keeper may look more aggressive).
To further investigate what type of information deWnesthe target location in far-aiming, we asked participants toslide a disk over the exact midpoint of a line that wasembedded in a Judd Wgure. We assumed that inXuences ofvisual context (i.e., the arrowheads) are indicative for theuse of relative metrics, pointing to contributions fromvision for perception rather than vision for action (Ganelet al. 2008). We hypothesized that if there is a role forvision for perception in far-aiming, then sliding accuracywill be biased by the illusion.
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Method
Participants
Twelve right-handed undergraduates of the University ofHong Kong (mean age 21.6 years, SD = 1.3) volunteered toparticipate. They were treated in accordance with the localinstitution’s ethical guidelines and gave written consentprior to the experiment.
Material
Participants were seated in front of a table (1.52 m in lengthand 1.37 m in width). Three visual targets were used, con-sisting of black lines embedded in a Judd Wgure with thearrowheads pointing to the right or to the left, or withoutarrowheads. Commonly, the arrowheads aVect perceptionof the midpoint of the line such that the perceived midpointis shifted to the right for arrowheads pointing to the left andvice versa. The lines, which were printed on a sheet ofpaper, measured 0.08 m in length and 0.02 m in width,while the arrowheads were 0.02 m in length and had aninclination of 45° to the line. The targets were placed in themiddle of the table or 0.02 m to the right or left of the mid-dle, at a distance of 1.2 m from the front edge of the table,where the participant was seated. A red block (0.01 £0.01 £ 0.01 m) served as the target during practice trials.A Chinese chequer disk (0.02 m in diameter and 0.01 m inheight) served as the projectile. Two pre-calibrated camerasof a QualisysTM 3-D motion capture system recorded thetrajectory of the disk at 100 Hz. ReXective tape made thedisk visible. There were two reference markers at the edgesof the table aligned with the target.
Procedure and design
The participants performed an action and a perception task.For the action task, the participants started with a series of100 practice trials. They were instructed to aim the disk atthe red target block by making a sliding action. Participantsobserved the experimenter produce the sliding action tobecome acquainted with how to hold, move and release thedisk. After the practice trials, the participants wereinstructed to propel the disk over the exact midpoint of thetarget line that was presented on the table top (Fig. 1). Theymade a total of 90 sliding actions to 3 diVerent target lines(i.e., line without arrowheads (‘control’), line with JuddWgure arrowheads pointing to the left (‘Judd left’) and linewith arrowheads to the right (‘Judd right’)) at three posi-tions (i.e., middle of the table and 0.02 m to the right or left,from the middle). The 9 conditions were presented inblocks of 10 trials, the order of which was randomizedacross participants. The participants propelled the disks one
at a time and at their own pace. Short rest intervals wereprovided between blocks.
For the perception task, the experimenter moved apointer slowly behind the line from the left to the right (andvice versa in the other half of the trials). The participantswere told to say ‘stop’ when they perceived that the pointerwas at the exact midpoint of the line. Participants wereallowed to alter their estimate if they had second thoughtsabout its correctness. The experimenter then marked theestimated midpoint on the sheet. A fresh sheet was used foreach estimate. The same 9 conditions (i.e., same target linesand positions) as in the action task were used. They werepresented in blocks with the order randomized across par-ticipants. Each condition was presented 4 times, resulting in36 trials. Finally, the order of the perception and actiontasks was counterbalanced between participants.
Data analysis
For the action task, the trajectory of the disk was used tocompute the location, where the disk crossed the target line.Sliding error was deWned as the diVerence (in mm) betweenthis location and the midpoint of the line. If absolute slidingerror was smaller than 10 mm (i.e., the disk touched themidpoint) a hit was scored, otherwise a miss to the left (i.e.,error <¡10 mm) or to the right (i.e., error >+10 mm) wasscored. For the perception task, the judgment error was
Fig. 1 Schematic representation of the experimental set-up (i.e., not toscale). Depicted is a Judd left Wgure in the mid position
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deWned as the diVerence (in mm) between the estimated andthe actual midpoint. We submitted both the sliding errorand the perceptual judgment error to an ANOVA (Target:control, Judd left and Judd right) with repeated measures.The number of hits and the number of misses to the left andright were examined using a MANOVA (Target: control,Judd left and Judd right) with repeated measures. Post-hoccomparisons were made using Tukey-HSD, and �p
2 wasused as the measure of eVect size. Finally, the diVerencebetween the Judd right and Judd left Wgures for the slidingand perceptual judgment errors served as indicators for theillusory bias in the action and perception tasks, respec-tively. T tests were used to assess whether the bias diVeredfrom zero.
Results
For the action task, the participants generally aimed to theleft of the midpoint of the target line, but less so when itwas embedded in the Judd left Wgure (Table 1). TheANOVA conWrmed that sliding error was signiWcantlyinXuenced by Target (F(2,22) = 4.07, p < 0.05, �p
2 =0.27).Post-hoc tests indicated diVerences in error between theJudd left and Judd right Wgures and between the Judd leftand control Wgures, but not between the Judd right and con-trol Wgures. Figure 2 presents the illusory bias for each indi-vidual participant. Ten out of twelve participants showed abias, with a mean bias of 6.4 mm (SD = 9.0 mm) that sig-niWcantly diVered from zero (t(11) = 2.46, p < 0.05).
Similar to sliding error, the MANOVA on the perfor-mance measures revealed a signiWcant eVect of Target(Wilks � = 0.54, F(6,40) = 2.37, p < 0.05, �p
2 =0.26). Sep-arate ANOVAs indicated that the illusion did not aVect thenumber of hits (F(2,22) = 1.86). The misses were signiW-cantly aVected by the illusion. Participants missed more tothe left of the midpoint when aiming at the Judd right andthe control Wgures than when aiming at the Judd left Wgure(F(2,22) = 5.54, p < 0.05, �p
2 =0.34). Conversely, partici-pants missed more to the right when aiming at the Judd leftand control Wgures than when aiming at the Judd rightWgure (F(2,22) = 4.00, p < 0.05, �p
2 =0.27) (Table 1).
For the perception task, the ANOVA indicated a signiW-cant main eVect of Target (F(2,22) = 94.5, p < 0.001,�p
2 =0.90). Post-hoc tests showed that the midpoint esti-mates for each of the Wgures diVered. The average illusorybias was 4.3 mm (SD = 1.3), which signiWcantly diVeredfrom zero (t(11) = 11.7, p < 0.001). Finally, a comparisonof the magnitudes of the illusory bias in the perception andaction tasks did not reveal a signiWcant diVerence(F(1,11) = 0.64). A Pearson-product correlation, however,failed to show a signiWcant relationship between the twobiases (r(12) = ¡0.24, p = 0.48).2
Discussion
This study provided evidence of a contribution to action byvision for perception. Sliding accuracy toward the midpointof the distant target showed a signiWcant illusory bias.3 Thedetermination of target location thus strongly depended oninformation that speciWes the midpoint in relation to itsvisual surrounding (i.e., arrowheads) in relative metrics,
Table 1 Mean (SD) for sliding error (mm), number of hits, misses toleft and right, and estimate error (mm)
Judd left Control Judd right
Sliding error ¡6.1 (9.3) ¡12.5 (8.3) ¡12.6 (3.9)
Hits 6.9 (2.6) 5.1 (3.1) 6.3 (2.4)
Miss to left 13.0 (3.3) 15.3 (2.9) 15.9 (1.4)
Miss to right 10.0 (2.7) 9.7 (2.7) 7.7 (2.3)
Estimate error 2.0 (1.1) ¡0.3 (1.0) ¡2.3 (0.6)
2 Exclusion of the two participants with a negative illusory bias inaction still resulted in a nonsigniWcant correlation (r(10) = 0.41,p = 0.12).3 In fact, only the Judd left Wgure diVered signiWcantly from the controlWgure. Perusal of the literature shows that this observation is notuncommon (e.g., Fleming and Behrmann 1998), but we found noexplanation for this asymmetry.
Fig. 2 The illusory bias in aiming for each individual participant.A bias occurred in all but two participants (i.e., 1 & 9)
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indicating that vision for perception can indeed be engagedin action. Prior work has already pointed to a role of visionfor perception in the choice of action goals and actionmodes (for overview see Milner and Goodale 2008). Thepresent study further delineates this role of vision for per-ception in action by suggesting that it also contributes to thedetermination of target location. This does not imply thatthe distinction between vision for perception and vision foraction becomes superXuous, nor does it prove that it iscorrect (Smeets et al. 2002; Vishton et al. 1999). EVects ofvisual context persist only when online movement controlis minimized or perturbed, such as in far-aiming or visualopen-loop conditions (Post and Welch 1996). In full-visionconditions, errors in perceived target location are rapidlyreduced during movement execution by vision for action.Ellis et al. (1999), for example, found that the location atwhich a bar was grasped was not systemically distorted bythe Judd illusion (see also Gentilucci et al. 1996).
Vickers (1992) recorded gaze in participants performingfar-aiming tasks like golf putting and basketball shooting.Information about the target, especially its exact locationwas picked up prior to, rather than during movement execu-tion. Vickers argued that the duration of the Wnal targetWxation reXects the time needed for movement parameteri-zation. She envisioned that target information stipulates theparameterization of the pre-programed movement kinemat-ics (see Glover 2004). In contrast, rather than prescribingthe movement kinematics, we argue that target locationinformation obtained by vision for perception acts as aboundary constraint on vision for action. It is within theseboundary constraints that vision for action instantaneouslysets up and controls the kinematics of the movement inreal-time (Westwood and Goodale 2003). Admittedly,however, we cannot distinguish the validity of these twoalternative accounts based on the present Wndings. Furtherwork is needed to determine how vision for perception andaction interact during the course of an action.
One critical factor that may inXuence the extent to whichvision for perception contributes to action is skill level(Gonzalez et al. 2008; van der Kamp et al. 2003, 2008). Animportant distinction between novice and skilled perform-ers is the degree of conscious control of the action. It isplausible that the more consciously an action is controlled,the more likely it is that it engages vision for perception.Gonzalez et al. (2008), for example, found that unfamiliarawkward grips were much more susceptible to a size-con-trast illusion than the precision grips that participants habit-ually used to grasp small objects. The present participantswere all novices.4 Hence, the illusory bias might have been
markedly smaller, had the participants been skilled shuZe-boarding players.
The illusory biases in the action and perception taskswere not identical nor were they correlated, suggesting thatthe tasks induced somewhat disparate contributions fromvision for perception. One possible distinction is that in theaction task the target location (i.e., midpoint of the line)was visually available relative to the visual surroundingsonly (i.e., endpoints of the line), while in the perceptiontask the moving pointer may have provided additional con-textual information. Previously, Post and Welch (1996)found that the illusionary bias in pointing to the midpoint ofa line within a Judd Wgure disappeared when a short linewas added to mark the midpoint. Yet, the moving pointer inthe present study does not necessarily provide more veridi-cal information for the midpoint. Hence, it is unlikely thatthis informational diVerence between the present taskscaused a larger reliance on relative metrics in the actionthan in the perception task. In any case, the present Wndingsdo underline the role of vision for perception in action forfar-aiming tasks that are not uncommon. It is akin to direct-ing a soccer penalty kick inside the uprights of the goalrather than at the uprights or placing a tennis serve in theback right corner of the service box rather than at the lines.It remains to be seen whether the present Wndings general-ize to targets that are visually more directly speciWed.Another observation that suggests disparate contributionsfrom vision for perception in the two tasks is that partici-pants slid the disk slightly to the left of the midpoint of theline. A similar leftward bias is reported for adults in linebisection tasks, where participants indicate the midpoint ofa line (McCourt and Garlinghouse 2000). Noticeably, thisleftward bias only presented itself during the far-aimingtask, but not when participants perceptually judged themidpoint of the line. This diVerence might be related tospace being perceived diVerently within or beyond theaction space. Longo and Lourenco (2006) demonstratedthat the leftward bias in a line bisection task disappeared, orshifted to the right, when the line is placed out of reach ofthe observer. However, when the participant used a tool(i.e., a stick), thereby expanding the action space, the left-ward bias also occurred for larger distances. Similarly, slid-ing a disk to a distant target might have expanded theparticipants’ action space, resulting in a leftward bias inaiming toward the target, but not when perceiving itsmidpoint.
In summary, mapping the contribution of vision forperception to action is conditional for understanding howvision for perception and vision for action interact. In thisrespect, we found an illusory bias when a projectile wasaimed toward a distant target, providing evidence for acontribution to action by vision for perception. Thismakes a great deal of sense given that determining target
4 This is suggested by the mean within-participant standard deviationfor the sliding error being much higher than for the perceptual judg-ment error (i.e., 40.1 mm vs. 9.9 mm).
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location in far-aiming (e.g., inside the base-line) is tightlylinked with identifying action goals (e.g., hitting the ten-nis ball down-the-line or cross-court) and selecting anappropriate action mode (e.g., fore- or back-hand), bothof which involve vision for perception. Further investiga-tion is needed to deWne how the two visual processesinteract.
Open Access This article is distributed under the terms of the Crea-tive Commons Attribution Noncommercial License which permits anynoncommercial use, distribution, and reproduction in any medium,provided the original author(s) and source are credited.
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